Modeling of organic pollutant destruction in a stirred2tank reactor by ozonation专业英语论文.docx

Modeling of organic pollutant destruction in a stirred2tank reactor by ozonationCHENGJiang1 , YANG Zhuo2ru1 , CHEN Huan2qin1 , KUO C. H. 2 , ZAPPI E.M. 2 Abstract :Destruction of organic contaminants in water by ozonation is a gas2liquid process which involves ozone mass transfer and fast irreversible chemical reactions. Ozonation reactor design and process optimizing require the modeling of the gas2liquid interactions within the reactor. In this paper a theoretical model combining the fluid dynamic and reaction kinetic parameters is proposed for predicting the destruction rates of organic pollutants in a semi2batch stirred-tank reactor by ozonation. A simple expression for the enhancement factor as our previous work has been applied to evaluate the chemical mass transfer coefficient in ozone absorption. 2 ,42dichlorophenol (2 ,42DCP) and 2 ,62DCP or their mixture are chosen as the model compounds for simulating , and the predicted DCP concentrations are compared with some measured data.Keywords : dichlorophenol destruction ; ozonation ; stirred2tank reactor ; enhancement factorIntroduction Because of the high oxidation potential of ozone (O3 ) , ozonation has been regarded as a promising method for drinking and waste water treatment. A wide range of organic pollutants in water can be degraded by O3 , O3 combined with H2O2 or UVlight , which are known as Advanced Chemical Oxidation Processes (AOPs) . Compared to the traditional treatment technologies , such as activated carbon adsorption or biodegradation , chemical oxidation with ozone offers the advantages ofgreater rate and extent of contaminant destruction. Although there are numerous reports (Hoigne , 1983 ; David , 1991) on the ozonation kinetics research regarding reaction rate constant , stoichiometric ratio and the identification of intermediates , application of these reaction kinetics to yield essential information for successful reactor and process design has not been received sufficient attention ( Yue , 1992) . This may be partly due to the lack of the chemical mass transfer coefficient of ozone in a specific reactor. It is well known that the mass transfer rate of a gaseous solute in absorption is enhanced by chemical reactions. The extent of this influence is expressed in terms of the enhancement factor , E , which is defined as the ratio of the mass transfer coefficient of the chemical absorption to that of physical absorption. In general , it is hard to determine the chemical mass transfer coefficient by experiment especially in absorption processes accompanied by complex reactions while the physical mass transfer coefficient may easily be obtained experimentally or from semi-empirical approaches. Based on the film theory Kuo (Kuo ,1982) proposed an iteration method for predicting the enhancement factor of mass transfer by ozone self2decomposition and ozonation reactions. Because the derived enhancement factor is an implicit expression , it is inconvenient in application to simulating the degradation rates of organic pollutants in an ozonation reactor. In this paper a simple explicit expression of the enhancement factor (Cheng , 2000) relating to the Danckwerts surface renewal model in ozone absorption with a first order ozone self2decomposition and a second order ozonation or a series of parallel ozonation reactions ( ajA + Bj pj Pj , j = 1 ,2 n) has been applied to predict the DCP destruction rate by ozonation in a semi2batch stirred tank reactor.1 Mathematical model1. 1 Destruction of one single organic pollutant in aqueous solution by ozonation When ozone is bubbled into a semi-batch stirred tank containing one organic pollutant solution , both gas and liquid phases can be assumed well mixed. The mass balance for ozone in the gas phase can be expressed as (Qiu , 1999) :where cA , G , cA , G,0 , cA , i and cA ,L represent the concentration of ozone in the gas bulk , in the influent stream of the reactor ,at the gas2liquid interface and in the liquid bulk respectively. H , uG , G are the liquid height in the reactor , the velocity ofgas phase and the gas holdup fraction , respectively , and t is the absorption time. kL as is the overall volumetric ozone physical mass transfer coefficient ( kL is the physical mass transfer coefficient , as is the specific interfacial area ) , E is the enhancement factor , and their product , kL as E , denotes the chemical mass transfer coefficient as discussed above. Because no dissolved ozone was detected in most experiments ( Kuo , 1982 ; Qiu , 1999) , i. e. cA ,L 0 , all the fast ozonation reactions can be assumed to complete within the liquid film. Then the depletion rate of the organic component in the liquid phase can be written as (Sotelo , 1990) :where cB ,L refers to the concentration of organic compound B in the liquid bulk , and a is the stoichiometric ratio of the ozonation reaction. In the above two equations the interfacial concentration of ozone cA , i can be expressed as cA , i = cA , G ×Sr. The solubility ratio of ozone Sr is 0. 21 0. 28 in the pH range of 5 9 (Qiu , 1999) and an average value 0. 24 is adopted here. A simple explicit expression of the enhancement factor in ozone absorption with ozone self2decomposition and a second order ozonation reaction was derived in our previous work (Cheng , 2000) based on the surface renewal model aswhere DA and DB are the diffusivity of ozone and organic B respectively. k is the second order reaction constant , kd is the first order ozone self2decomposition reaction constant. It should be noted that Eq. (2) is valid only if the following condition holds M = Md + M1 1 - ( E - 1) /Q > 4.1. 2 Destruction of the mixture of organic pollutants in aqueous solution by ozonation When two or more contaminants are present initially in the liquid , the absorption of ozone is accompanied by parallel ozonation reactions. If the competition between these reactions is considered to be dependent only on the reaction constants ,the depletion rate of organic component Bj in the liquid phase can be derived asThe mass balance for ozone A in the gas phase is the same as Eq. (1) .For the enhancement factor in ozone absorption with parallel ozonation reactions , an approximate expression can also be deduced relating to the surface renewal model as (Cheng , 2000)where Ej represents the supposed enhancement factor in ozone absorption accompanied by a first order ozone self2 decomposition reaction and a second order ozonation reaction of a single reactant Bj .2 Simulating and experimental results 2 ,42DCP and 2 ,62DCP isomers were chosen as the model compounds for simulatingwhich are the least and most reactive species with ozone in the DCP isomers respectively. Dichlorophenols have been widely used in the production of pesticides , dyes and other industrial chemicals. They are listed among the 65 priority pollutants by the EPA in the Clean Water Act of 1977. The water quality criteria recommended by the EPA for DCP is 0. 04 to 0. 5gL - 1 . For ozone absorption in a single DCP solution , the enhancement factor E , the concentrations of DCP in the liquid bulk and ozone in the gas phase , cB ,L , cA , G , can be predicted theoretically as shown in Fig. 1 and Fig. 2. by combining Eqs. (1) , (2) and (3) and applying the numeric method of MATLAB ODE program. Some experimental results by Qiu (Qiu ,1999) are also presented in Fig. 1a and Fig. 2a. The stirred2tank reactor used in the experiment , as sketched in Fig. 3 , is composed of a glass cylinder with inner diameter of 15 cm held between the top and bottom circular stainless steel plates by eight screw rods. A four-bladed baffle of stainless steel is inserted into the reactor to increase the turbulence of the gas and liquid phases. The stirrer is a turbine impeller with 6 blades. The gas sparger , a fritted disc with a porosity of 40 to 60m ,is connected to a glass tube through which the ozone gas is bubbled into the tank. The initial volume of the solution is 3 liters in each experiment. Under the experimental condition the stirring Reynolds number of the solution is evaluated greater than 10000 , indicating perfect mixing has been achieved.Fig. 1 Prediction of enhancement factor and concentration of ozone absorption in 2 ,42DCP solution(Stir speed : 200 r/min , cA , G,0 = 0. 0002 mol/L , cB ,L ,0 = 0. 0005 mol/L , a = 2 , Sr = 0. 24)Curve No.Key *pHq , L/mink1,L/(mol·s)kd , s - 1kL a , s - 1kL ×104mPsG1511.2 ×1060.00030. 011343. 80. 012711. 1 ×1080.0010. 011343. 80. 013+917. 5 ×1080.20. 011343. 80. 01471.51. 1 ×1080.0010.016054.760. 013* experimentalFig. 2 Prediction of enhancement factor and concentration of ozone absorption in 2 ,62DCP solution(Stir speed : 200 r/min , cA , G,0 = 0. 0002 mol/L , cB ,L ,0 = 0. 0005 mol/L , a = 2 , Sr = 0. 24)Curve No.Key *pHq , L/mink1, L/(mol·s)kd , s - 1kL a , s - 1kL ×104 , mPsG1514.0×1070. 00030. 011343.80.012711.5×1090. 0010. 011343.80.01* experimentalIt can be seen from Fig. 1a and Fig. 2a that the predicted concentrations agree well with the experimental results at the early period of absorption (up to 90 % consumption of DCPs) in the pH range of 5 9. But at pH 5 , significant deviation appears with ozone absorption in 2 ,42DCP solution. This is because at pH 5 the ozonation reaction constant of 2 ,42DCP decreases and the criteria of M < 4 may be not satisfied anymore , which means ozonation reactions have not completed within the liquid film and may extend to the liquid bulk. On the other hand , the mathematical model failed to simulate the concentration behavior when DCPs decrease to a certain low level , i. e. lower than 10 %of initial concentration , and predicts a shorter treatment time required for DCP removal than that measured in experiments. This is reasonable because the model neglects the side reactions between ozone and the uncertain intermediates when oxidation of DCPs nearly completes. Part of ozone may be consumed by these intermediates and result in the longer treatment time in a real absorption process. Both experimental and prediction results in Fig. 1a indicate that as the pH increases from 7 to 9 , there are little changes in the destruction rate of 2 ,42dichlorophenol because of the limitation of ozone mass transfer. However when the gas flow rate qincreases from 1 to 1. 5 LPmin at a fixed stir speed of 200 rPmin , the oxidation rate increases greatly due to the higher ozone mass transfer rate and ozone dosage.